Su(z)12, a highly conserved polycomb group (PcG) gene, suppresses position-effect variegation (PEV), and is essential for embryonic and germ cell development in Drosophila. Orthologs of Su(z)12 in Arabidopsis control the timing of flowering and seed development, processes that are vital to the reproduction of the plant. Furthermore, a translocation creating a fusion product between Su(z)12 and another zinc finger gene are common to endometrial stromal sarcomas. Despite its apparent function in these important biological processes, very little is known about the mechanisms by which Su(z)12 acts. The objectives of this proposal are to first examine the effects of Su(z) 12 deletion in mouse embryonic and germ line development, and then to determine whether or not these phenotypes are mediated by dysfunction in heterochromatin assembly. Su(z)12-null and conditional germ line null alleles will be created and analyzed at the phenotypic level. The Su(z)12-null animals will provide tissues and cells that will be used in molecular assays of heterochromatin assembly and genomic stability to determine whether these processes are disrupted in Su(z)12 mutant mice.